How to automatically update /etc/apt/sources.list with source URIs on Ubuntu 16.04?

The following command worked fine on Ubuntu 15.10:

sudo apt-get build-dep emacs24

However, on Ubuntu 16.04 I get the following error when running it:

Reading package lists... DoneE: You must put some 'source' URIs in your sources.list

In 15.10 all lines (around 10 in number) with deb-src in /etc/apt/sources.list where uncommented, whereas in 16.04 the corresponding lines where commented out. For example, here are 4 lines from my current sources.list:

## Major bug fix updates produced after the final release of the## distribution.deb http://no.archive.ubuntu.com/ubuntu/ xenial-updates main restricted# deb-src http://no.archive.ubuntu.com/ubuntu/ xenial-updates main restricted

What is the reason for this change?

Next, I would like to avoid manually edit /etc/apt/sources.list each time I do a reinstallation of Ubuntu. How can this (uncommenting the deb-src lines) be done automatically?